1. Field of the Invention
This invention relates to activated carbon pellets and activated granules for automotive emission control canisters with improved dusting characteristics. In particular, this invention relates to activated carbons susceptible to dust attrition due to abrasion where dusting can result in loss of product and often cause other problems related to its use in automotive emission control canisters.
2. Description of Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Active carbon long has been used for removal of impurities and recovery of useful substances from liquids and gases because of its high adsorptive capacity. Generally, “activation” refers to any of the various processes by which the pore structure is enhanced. Typical commercial activated carbon products exhibit a surface area (as measured by nitrogen adsorption as used in the B.E.T. model) of at least 300 m2/g. For the purposes of this disclosure, the terms “active carbon” and “activated carbon” are used interchangeably. Typical activation processes involve treatment of carbon sources) such as resin wastes, coal, coal coke, petroleum coke, lignites, polymeric materials, and lignocellulosic materials including pulp and paper, residues from pulp production, wood (like wood chips, sawdust, and wood flour), nut shell (like almond shell and coconut shell), kernel, and fruit pits (like olive and cherry stones) either thermally (with an oxidizing gas) or chemically (usually with phosphoric acid or metal salts, such as zinc chloride).
Chemical activation of wood-based carbon with phosphoric acid (H3PO4) is disclosed in U.S. Pat. No. Re. 31,093 to improve the carbon's decolorizing and gas adsorbing abilities. Also, U.S. Pat. No. 5,162,286 teaches phosphoric acid activation of wood-based material which is particularly dense and which contains a relatively high (30%) lignin content, such as nut shell, fruit stone, and kernel. Phosphoric acid activation of lignocellulose material also is taught in U.S. Pat. No. 5,204,310 as a step in preparing carbons of high activity and high density.
Also, U.S. Pat. No. 4,769,359 teaches producing active carbon by treating coal cokes and chars, brown coals or lignites with a mixture of NaOH and KOH and heating to at least 500° C. in and inert atmosphere. U.S. Pat. No. 5,102,855 discloses making high surface area activated carbon by treating newspapers and cotton linters with phosphoric acid or ammonium phosphate. Coal-type pitch is used as a precursor to prepare active carbon by treating with NaOH and/or KOH in U.S. Pat. No. 5,143,889.
Once the activated carbon product is prepared, however, it may be subject to some degradation before and during its use. Abrading during materials handling and actual use of such activated carbon results in loss of material in the form of dust. Such “dusting” of the product is a function of its relative hardness and its shape, as well as how it is handled in the plant—in moving it into and out of plant inventory, in loading for transport and in off-loading by the receiver, and how it is handled by the receiver to place the product into use. In certain applications, such as employment in canisters in automobiles where the activated carbon is subject to constant vibration and may have to withstand collision, product degradation by dusting continues through the life of the product.
The hardness of an activated carbon material is primarily a function of the hardness of the precursor material, such as a typical coal-based activated carbon being harder than a typical wood-based activated carbon. The shape of granular activated carbon also is a function of the shape of the precursor material. The irregularity of shape of granular activated carbon, i.e., the availability of multiple sharp edges and corners, contributes to the dusting problem. Of course, relative hardness and shape of the activated carbon are both subject to modification. For example, U.S. Pat. Nos. 4,677,086, 5,324,703, and 5,538,932 teach methods for making pelleted products of high density from lignocellulosic precursors. Also, U.S. Pat. No. 5,039,651 teaches a method of producing shaped activated carbon from cellulosic and starch precursors in the form of “tablets, plates, pellets, briquettes, or the like.” Further, U.S. Pat. No. 4,221,695 discloses making an “Adsorbent for Artificial Organs” in the form of beads by mixing and dissolving petroleum pitch with an aromatic compound and a polymer or copolymer of a chain hydrocarbon, dispersing the resultant mixture in water giving rise to beads, and subjecting these beads to a series of treatments of removing of the aromatic hydrocarbon, infusibilizing, carbonizing, and finally activating.
Despite these and other methods of affecting activated carbon hardness and shape, however, product dusting continues to be a problem in certain applications. For example, in U.S. Pat. No. 4,221,695, noted above, the patentees describe conventional beads of a petroleum pitch-based activated carbon intended for use as the adsorbent in artificial organs through which the blood is directly infused that are not perfectly free from carbon dust. They observe that some dust steals its way into the materials in the course of the preparation of the activated carbon, and some dust forms when molded beads are subjected to washing and other treatments. The patentees note that the application of a film-forming substance to the surface of the adsorbent “is nothing to be desired,” because the applied substance acts to reduce the adsorption velocity of the matters to be adsorbed on the adsorbent and limit the molecular size of such matters being adsorbed.
Subsequently, in U.S. Pat. No. 4,476,169, the patentees describe a multi-layer glass window wherein vapor between the glass sheets is adsorbed by a combination of a granular zeolite with granular activated carbon having its surface coated with 1–20 wt % synthetic resin latex. The coating of the activated carbon is described as greatly inhibiting the occurrence of dust without substantially reducing the absorptive power of activated carbon for an organic solvent.
Automotive canisters for controlling fuel vapor emissions use activated carbon in either granular or pelletized forms. Activated carbons, regardless of their form and size, contain some portion of smaller particles, or dust, which can be problematic for valves and filters associated with the canister. This dust can present a nuisance at canister filling operations that dispense and convey bulk quantities of activated carbon. Reduction of dust can reduce the likelihood of valves and filters on canisters becoming partially or fully blocked and relieve the nuisance issues at canister filling locations. Dust issues can arise from either initial dust present as a result of sizing and screening inefficiencies or from dust generated by the action of pellets and granules against one another, which can be quantified as a dust attrition rate.
In addition to dust suppression, the coatings can provide a means of colorizing activated carbon so that it has an appearance besides the customary black, as described in earlier patent notes. Color can serve as a means of identifying different grades and/or manufacturing dates for activated carbon. Different color coatings can provide an effective means of differentiating between different grades, such as low bleed pellets and high capacity pellets that are used in a dual-fill canister that has high capacity and low bleed emissions. Color can also be used as a means of identifying the year the activated carbon was manufactured. Another use of color coating is for quality assurance. For example an automotive manufacturer could demand red BAX 1500 as a means of assuring that a certain manufacturer's product is used.